There is a real need for a practical, low-cost solar energy conversion system which has simple long-lived mechanical components, and functions with a minimum of breakdowns and replacement of individual parts.
Because the state of the art in silicon photovoltaic solar cells is far away from the cost competitive marketing stage, it appears that an alternate, low-cost mechanical solar thermodynamic conversion method is worthy of development, and eventual market acceptance.
In addition to their inordinate high cost, even in sizable quantities, silicon solar cells are fragile and are subject to breakage during violent storms and their full life expectancy has yet to be established in wide scale field installations. When the cost of these solar cells is eventually lowered to a wide scale commercial acceptance level, the above design problems will probably still exist to some degree.
In view of the various shortcomings of the solar photovoltaic conversion method, a simplier thermodynamic conversion arrangement which has already been tried and established, appears to be more promising, since it offers lower cost, and better over-all ruggedness and relaibility.
The key component for any type of solar thermodynamic conversion system will be the reflective concentrator unit, either the dish or linear parabolic concentrator, since large diameter lenses are economically impractical. Most of the attention in the experimental development of solar concentration has gone to the dish or circular type of parabolic reflector.
The attraction of the dish parabolic reflector is difficcult to rationalize from an objective design standpoint, since it offers a generally poor concentration area "spot", to dish exposure area ratio, and a very high spot temperature will usually pose more of a design problem than the value of the end application.
The current academic effort to match the disc parabolic reflector with a high output, water-cooled silicon solar cell must be viewed with some misgivings, and as a design mismatch when all the various design factors are thoroughly considered.
The very high temperature spot of the dish parabolic reflector would be better applied to the hot side of a moderately rated Stirling cycle engine, which would drive an alternator for electrical output, that on a silicon solar cell of far lower corresponding wattage output. Although the comparitive cost of both arrangements would favor the photovoltaic means, it is probable that the cost per watt of the solar mechanical to electrical conversion method would cost about one third of an equivalent 10 KW/hr photovoltaic system installation.
A very attractive middle ground between one sun (no concentration), and very high 500 sun (500:1 concentration ratio), would be adoption of linaer parabolic reflectors, with concentration ratios between about 20:1 and 50:1. The linear parabolic reflectors would be compatibly practical for both photovoltaic conversion and thermodynamic mechanical to electrical solar conversion methods.
Another useful design feature of the linear parabolic reflector (L.P.R.), is that they offer a good concentration "strip", area to panel exposure area ratio, which makes this type of concentrator a natural match with the flash boiler pipes of the hot water steam engine alternator system. The narrow long solar concentration strip will naturally coincide with the relatively long flash boiler pipe, to flash the hot water over to steam within a reasonal total lenght of joined pipe lengths.
The L.P.R.'s will also be practical for solar photovoltaic conversion systems because of the improved concentration strip area to panel exposure area ratio previously mentioned, and the fact that the lower concentration temperatures allow the use of lower cost photovoltaic solar cells..
Of the two possible solar conversion means covered, the matching of the L.P.R.'s with the flash boiler pipes is projected as the most promising method, based on the factors of first and operating costs, reliability, ruggedness, and maintainability.
Another immediate point in favor of the mechanical to electrical conversion means is that all of the cost factors can be accurately predetermined, unlike the solar photovoltaic cells, where the present high costs are prohibitive for most installations, with the future cost picture uncertain and showing little sign of imminent cost reduction.
The cost of fabricating the linear parabolic reflectors will be substantially less than for a corresponding circular dish parabolic reflector, because simple linear forms can be used to produce the L.P.R.'s compared with the more complex steps required for the dish paraboloids.
The presently advocated quadrant four deep concave, linear solar concentrator panel differs from the earlier classic linear parabolic reflectors in that the earlier L.P.R.'s provided solar concentration in the two lower quadrants and only in one dimension, the width at the focal line. The earlier type of L.P.R. used to heat water into steam through a focal line pipe was shallow with a limited concentration ratio of about 5:1.
This latest type of L.P.R. provides solar concentration in all four quadrants at the focal zone, or all around solar heating in two dimensions.
The additional concentration quadrants are provided by the inclusion of uniformly concave reflector extension sections onto the lower base parabolic reflector cross-section. These reflective extensions are smoothly blended into the base parabolic section, and both add about one quadrant of solar concentration at the flash boiler pipe.
The fourth and completing concentration quadrant is provided by top linear convex lens sections, which concentrate solar rays directly onto the top of the flash boiler pipe(s).
The solar rays that fall normal to the solar panel surface are concentrated in two dimensions, width and height, and since the panel is symmetrical about the vertical centerline, all around solar concentration is achieved when the top linear lens is also utilized. When the flash boiler pipe is placed at the normal focal line of the base parabolic reflector section, the solar rays are concentrated on the bottom width from the lower parabolic section, and the height, on both sides from the two side concave extension sections.
Although it is a logical design point to utilize a large size width of full parabolic reflector cross-section, for large solar concentration ratios coupled with a small size of steam boiler pipe. There are three reasons for not following this approach.
The first, and most important point is that the focal line of a para bola in linear form, is always in line with the two lateral end lines, by geometric definition, so that desirable solar concentration above the focal line is not possible. The second reason is that the basic parabolic shape will not have the best possible structural section modulus, when used for an elongate solar panel.
The final reason against the full parabola is that the solar rays must now focus on an elongate vertical surface, rather than a a (point) line, since the solar rays are concentrated on the flash boiler pipe which has height, in this geometric concentrator application.
A major design consideration in the application of any type of solar concentrator panel is that the vertical axis centerline must always be kept in line with the normal solar rays at any given time. The solar rays must fall normal to the horizontal axis of panels, so that symmetrical or uniform solar ray distribution is achieved. The failure of a solar concentrator panel to remain vertically lined up with the solar rays within about three degrees will result in an inordinate dropoff in the temperature of the flash boiler pipe, for any given solar intensity.
Essentially, this new type of four quadrant, deep concave, linear solar concentration panel requires the adoption of a new composite geometric cross-section, in order to achieve the desired ends of a practical, low-cost solar concentration method, for several solar conversion applications.